Alkaline storage batteries, which can be repeatedly charged and discharged, have been widely used as power sources for portable equipment. In recent years, in particular, nickel-metal hydride storage batteries, which have a high energy density and are relatively environmentally friendly, are dominant in the marketplace, and the demand therefor is rapidly growing in the fields that require high power output such as power tools and hybrid electric vehicles (HEV).
In positive electrode plates for alkaline storage batteries, porous metal substrates are preferably used as the core material because they are easy to be filled with a material mixture paste composed of an active material, and the rolling step after drying of the material mixture is easily performed. Further, improvement in capacity density can be expected. Particularly, foamed nickel substrates, which are produced by electroplating or electroless plating a urethane sheet with nickel, followed by baking to remove carbon components, are widely used as porous metal substrates.
High power output alkaline storage batteries have the following configuration to improve current collecting efficiency. On a strip-shaped electrode is formed an unfilled portion, where a material mixture composed of an active material is not filled, along one side of two longitudinal sides thereof. A positive electrode and a negative electrode each having this structure are spirally wound with a separator interposed therebetween to form a cylindrical electrode group, whereby the unfilled portion of the positive electrode is positioned at one end of the electrode group and the unfilled portion of the negative electrode is positioned at the other end of the same. By welding current collector plates thereto, it is possible to efficiently collect current from the electrodes.
Various attempts have been made to develop a method for producing a positive electrode plate for an alkaline storage battery, some of which are listed below.
(i) A method in which a porous metal substrate in a hoop shape is continuously fed into a vessel holding a material mixture paste composed of an active material so as to fill the paste into the substrate, which is then allowed to pass through a roll smoother to smoothen the surface of the substrate filled with the paste, followed by drying and rolling (see Japanese Laid-Open Patent Publication No. Hei 1-163965).
(ii) A method in which a material mixture paste is sprayed to a porous metal substrate with a high pressure from a nozzle to fill the paste into the substrate, which is then allowed to pass through a slit to remove excess paste therefrom, followed by drying and rolling.
(iii) A method in which a material mixture paste is filled from one surface of a porous metal substrate such that most part of the other surface is not filled with the paste, followed by drying and rolling. In this method, preferably, the material mixture paste is filled from one surface of the porous metal substrate such that the other surface is not filled with the paste at all (see Japanese Laid-Open Patent Publications Nos. Hei 9-106814 and Hei 9-27342).
(iv) A method in which a material mixture paste is sprayed to both surfaces of a porous metal substrate in a hoop shape from nozzles each positioned close to each surface of the substrate so as to fill the paste into the substrate while the substrate is moved in the longitudinal direction thereof. The distance between the nozzle and the substrate is set to 1.0 mm or less (see Japanese Laid-Open Patent Publication No. Hei 9-106815).
In the proposals of Japanese Laid-Open Patent Publications Nos. Hei 1-163965 and Hei 9-106814, the entire porous metal substrate is filled with the material mixture paste. However, positive electrode plates for alkaline storage batteries need to have an unfilled portion where a material mixture paste is not filled (i.e. exposed portion of a substrate) to which a current collector plate is welded (see Japanese Laid-Open Patent Publication No. 2000-113881). Accordingly, it is necessary to remove the material mixture having been filled in the substrate.
In view of this, Japanese Laid-Open Patent Publication No. 2002-75345 proposes to press a porous metal substrate filled with a material mixture such that protrusions (ribs) are formed, to which ultrasonic vibration is applied to remove the material mixture from the protrusions, after which the protrusions are utilized as unfilled portions to which a current collector plate is welded. This method, however, is accompanied by problems such as more steps, more loss of active material and high costs.
Because the amount of a material mixture paste filled in a substrate depends on the porosity of the substrate, it is difficult to fill a constant amount of material mixture paste into a substrate using the proposals of Japanese Laid-Open Patent Publications Nos. Hei 1-163965 and Hei 9-106814, and a variation in the paste filling rate is caused. The paste filling rate is defined by the ratio of the volume of a paste filled into a substrate to the volume of pores of the substrate.
Further, according to the proposals of Japanese Laid-Open Patent Publications Nos. Hei 1-163965 and Hei 9-106814, because bubbles are generated when a paste is filled into a substrate, only a paste filling rate of about 90 to about 95% can be achieved at most and the porous metal is exposed on the substrate surface. As a result, metal burrs are easily formed when electrode plate is cut into a predetermined size. Also, a short circuit is likely to occur due to the exposed metal when the positive electrode and a negative electrode are spirally wound with a separator interposed therebetween to form an electrode group. In order to prevent the above problems, a thick separator should be used, which makes it difficult to achieve a high capacity battery.
According to the proposal of Japanese Laid-Open Patent Publication No. Hei 9-27342, it is possible to spray a constant amount of paste from a die, which significantly reduces the variation in paste filling rate in the longitudinal direction of the substrate. However, this is not practical because the paste filling rate is very low and therefore the electrode plates obtained after rolling have different thicknesses. As for battery performance, when metal is exposed on the entire one surface of an electrode plate, a part of metal is embedded in an adjoining separator, which shortens the distance between the positive and negative electrodes, resulting in a large amount of self discharge.
The proposal of Japanese Laid-Open Patent Publication No. Hei 9-106815 also requires the removal of the material mixture having been filled in the substrate in order to create an unfilled portion (i.e. exposed portion of a substrate), which results in high costs.
From the viewpoint of increasing productivity, it is proposed to form, in a substrate, an unfilled portion where a material mixture is not filled by applying a material mixture paste on the substrate in a strip pattern. Devices for performing such stripe application are also proposed. For example, Japanese Laid-Open Patent Publication No. 2000-233151 proposes a device equipped with a means to adjust a slit gap. Such stripe application is effective when a material mixture paste is applied on a substrate made of a metal foil. However, it is not always effective because, when a material mixture paste is filled into a porous metal substrate in a stripe pattern, the material mixture paste may easily enter the unfilled portion.
As mentioned earlier, porous metal substrates such as foamed nickel substrates are widely used for positive electrode plates for alkaline storage batteries. However, conventional porous metal substrates are costly to produce, and it is difficult to further improve the current level of the filling rate.
Although porous metal substrates having a lower weight per unit area than conventional ones can be produced at a relatively low cost, the use of a substrate having a low weight per unit area reduces current collecting efficiency, leading to a decrease in high rate discharge performance and active material utilization rate.
Further, when a material mixture paste is filled into a porous metal substrate having a low weight per unit area in a stripe pattern, the entering of the material mixture paste into the unfilled portion is facilitated, causing a great loss of active material. When a current collector is welded to the unfilled portion where the material mixture has entered, a weld defect may be caused by sparks or the like, reducing the strength of the welded portion. The material mixture having entered the unfilled portion can be removed completely, but it is inefficient.